Morphological differentiation driven by adaptive divergence among and within islands in the snail-feeding carabid beetle Carabus blaptoides

Geographical isolation among islands, and among regions within islands, can promote adaptive radiation. However, it remains unclear how isolation at these two spatial scales (among islands and within island) contribute to overall adaptive divergence, particularly during the early stages of adaptive radiation in taxa that gradually expand their distribution across continents or continental islands. In this study, we aimed to elucidate the geographical conditions during the initial stages of adaptive radiation in the geographically polytypic, snail-feeding carabid beetle Carabus blaptoides, which is distributed across the Japanese Archipelago and has diverged into eight subspecies. We reconstructed ancestral geographical distributions, inferred ancestral character states, and conducted Ornstein–Uhlenbeck analyses. Our reconstruction revealed that body shape diverged not only between the main island and the peripheral islands, but also between the eastern and western regions of the main island. Ornstein–Uhlenbeck analysis further suggested that adaptive divergence in body shape has proceeded towards distinct optima on peripheral islands and in different regions of the main island. These findings highlight that adaptive divergence both among islands and within island have played crucial roles in the early stages of adaptive radiation in large continental island systems.